1. Technical Field
This invention relates to audio systems and, more specifically, to systems for selectively connecting speech circuits to an audio line in response to voice signals.
2. Description of the Prior Art
Major companies are beginning to consider teleconferencing as a cost effective way of communicating among personnel at dispersed locations and thereby reduce the need for business travel. In a teleconferencing arrangement, a number of conferees at a location are placed in communication with a number of conferees at one or more remote locations via a telephone connection. The quality of the transmission between the separated groups of conferees is generally dependent upon the position of each conferee with respect to a microphone and loudspeaking device at each location. With a single microphone and loudspeaking device in the conference location room, the transmission is subject to degradation because some of the conferees are generally at a greater than optimum distance from the microphone and loudspeaking device.
It is well known to use a plurality of microphones appropriately spaced at each conferee location such as a conference room to improve the quality of the conference system. The microphone outputs are summed and the summed output is applied to the communication links between locations. In such an arrangement, each conferee can be within an acceptable distance from one of the microphones, whereby speech pickup is of relatively good quality. With all microphones turned on at one time, however, several undesirable effects occur. The total noise pickup is much greater than for a single microphone. The artificial reverberation effects occasioned by the delayed signal pickup from the more remote microphones severely lower the quality of the conference transmission. Further, electroacoustic instability can easily result from the plurality of the always turned on microphones. It is therefore desirable and known in the art to provide a switching arrangement which permits only that microphone closest to the talking conferee to be active so that reverberation and noise pickup are minimized.
Such an arrangement is commonly known as a "voting circuit." In the "voting circuit" arrangement, the loudest talker can capture control and lock out the other conferees at his location. This automatic switching between microphones responsive to the highest speech level microphones, however, may also result in transmission interruptions which adversely affect intelligibility and can result in unwanted interference occasioned by transient room noise. For example, a loud noise at one of the conference locations may completely turn off the controlling microphone. Further, since only one microphone is operative at a time, transfer of control from one microphone to another such as occasioned by the talking conferee moving from one position to another in a room location can result in speech transmission of varying quality, interruptions in transmission, and reverberation effects which vary with the talking conferee's position.
Various teleconferencing arrangements have been proposed and used heretofore for selecting a single microphone of a plurality of conferee microphones and for transmitting the signal from only the selected microphone. An example of such an arrangement is seen in U.S. Pat. No. 3,730,995, issued to M. V. Matthews on May 1, 1973. In this arrangement, each of a plurality of microphones is associated with a speech detector and a relay. In response to voice signals from one of the microphones, an associated speech detector activates its relay which connects the microphone to an audio line and generates a signal inhibiting the other relays. Another example is seen in U.S. Pat. No. 3,755,625, issued to D. J. Maston on Aug. 28, 1973. This patent discloses a multimicrophone-speakerphone arrangement using a comparator in combination with logic circuitry for selecting a microphone with the greatest output and connecting it to the speakerphone input while simultaneously disconnecting the other microphones. While these arrangements have been satisfactory in minimizing the degradation of the speech signals due to reverberation and noise pickup, it is nevertheless desirable to make the microphone selection technique appear to occur in as normal a manner as possible. That is, not only should the microphone voter recognize and respond to the loudest conferee in the room as do the other conferees, but it should also allow other conferees who speak simultaneously to be heard but at a lower level. And it is also desirable to avoid the syllabic clipping that occurs when a microphone turns on from the full off condition.